The class of foams known as low density rigid polyurethane or polyisocyanurate foam has utility in a wide variety of insulation applications including roofing systems, building panels, refrigerators and freezers. The methods of producing polyurethane and polyisocyanurate foams are generally known and consist in general of the reaction of an organic polyisocyanate and a polyol or mixture of polyols in the presence of a volatile blowing agent, which is caused to vaporize by the heat liberated during the reaction of isocyanate or isocyanurate and polyol. This reaction can be enhanced through the use of amine and/or other catalysts as well as surfactants. The catalysts ensure adequate curing of the foam, while the surfactants regulate and control cell size.
The foam industry has historically used liquid fluorocarbon blowing agents such as trichlorofluoromethane (CFC-11) and 1,1-dichloro-1-fluoroethane (HCFC-141b) because of their ease of use in processing conditions. Fluorocarbons act not only as blowing agents by virtue of their volatility, but also are encapsulated or entrained in the closed cell structure of the rigid foam and are the major contributor to the low thermal conductivity properties of rigid urethane foams. The escape of certain fluorocarbons, most notably chlorofluorocarbons, to the atmosphere is now recognized as potentially contributing to the depletion of the stratospheric ozone layer and to global warming. In view of the environmental concerns with respect to chlorofluorocarbon blowing agents, it is now generally accepted that it would be more desirable to use hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) rather than the chlorofluorocarbons. Exemplary hydrofluorocarbons suitable for use as blowing agents in the preparation of polyurethane and polyisocyanurate foams are 1,1,1,3,3-pentafluoropropane, 1,1,1,2-tetrafluoroethane and 1,1,2,2-tetrafluoroethane (known in the art as HFC-245fa, HFC-134a, and HFC-134, respectively). These materials, however, possess a lower boiling point and a correspondingly higher vapor pressure than the prior art materials. This difference becomes significant when any of HFC-245fa, HFC-134a or HFC-134 are incorporated into the B-side of a foam formulation that is either transported in or used directly from a low pressure rated container.
The use of a fluorocarbon as the preferred commercial expansion or blowing agent in insulating foam applications is based in part on the resulting k-factor associated with the foam produced. K-factor is defined as the rate of transfer of heat energy by conduction through one square foot of one inch thick homogenous material in one hour where there is a difference of one degree Fahrenheit perpendicularly across the two surfaces of the material. As is generally known in the art, the cell gas composition of the foam at the moment of manufacture does not necessarily correspond to the equilibrium gas composition after aging or sustained use. The gas in a closed cell foam frequently exhibits compositional changes as the foam ages leading to such known phenomena as increase in thermal conductivity or loss of insulation value (both measured in terms of k-factor) and thermal aging. Since the utility of closed-cell polyurethane-type foams is based, in part, upon their thermal insulation properties, it would be advantageous to identify materials which lower the k-factor of foams and reduce thermal aging of the foams over time.
Methods for improving the k-factor and/or thermal aging characteristics of polyurethane foam are disclosed in U.S. Pat. Nos. 5,696,306 and 5,837,742. There remains a need in the art for polyurethane and polyisocyanurate foams prepared from a blowing agent comprising a hydrofluorocarbon selected from the group consisting of HFC-245fa, HFC-134a, HFC-134 and mixtures thereof, which foams have improved processibility, k-factor and/or thermal aging characteristics. The instant invention meets this need by providing additives that reduce the vapor pressure of HFC-245fa and HFC-134a and/or reduce the k-factor of foams produced from blowing agents comprising these hydrofluorocarbons.